1. Field of the Invention
The present invention relates to a process for preparing saturated carboxylic acids having from one to four carbon atoms, and also an apparatus for carrying out the process.
2. The Prior Art
It is known that acetic acid can be prepared by gas-phase oxidation of C.sub.4 -hydrocarbons in the presence of a catalyst. Most descriptions provide for the reaction gas mixture to be passed once over the catalyst, for the acetic acid formed to be separated off by condensation and for the remaining gas to be discarded. For example, U.S. Pat. No. 3,917,682 describes a procedure in which acetic acid is obtained by butane and/or butene oxidation in the presence of a Ti/V catalyst having a high proportion of rutile. The acetic acid is isolated by partial. condensation of the reaction mixture. The remainder of the reaction gas is not recirculated. Such processes have to achieve a high butene conversion on only one pass through the reactor, which can be successfully achieved with small yields or low space-time outputs. For this reason, it has not yet been possible to find an economically satisfactory process on this basis.
U.S. Pat. No. 4,146,734 discloses that the gas-phase oxidation of butene to acetic acid can be carried out in the presence of a catalyst comprising lanthanide compounds. A method of isolating the acetic acid and further useful materials formed during the gas-phase oxidation is not disclosed.
DE-A 2,149,752 and DE-A 1,279,011 describe processes for the catalytic gas-phase oxidation of butene to acetic acid in the presence of specific catalysts. A disadvantage of this procedure is that in the indicated recirculation of the uncondensable proportion of the reaction gas, the formic acid obtained as a useful material decomposes. DE-A 1,921,503 refers to the possibility of recirculating the unreacted proportion of the reaction mixture to the reactor in the preparation of acetic acid by means of catalytic gas-phase oxidation of butene. However it is expressly stated that a circulating gas process is uneconomical.
A process was developed to the pilot scale by Chemische Werke Huls and described in various publications (R. P. Lowry, A. Aguilo, Hydrocarbon Processing, 10, (1974), 103; PEP Report No. 37A (1973)). This provides for the direct, untreated recirculation of 4/5 of the gas mixture leaving the reactor. In this embodiment, part of the reaction product is circulated without removal of the acids and only part is separated off for isolating the acetic acid. In this process, there is considerable accumulation of organic acids in the reaction gas, as a result of which both acetic acid and formic acid are obtained in an unsatisfactory yield.
WQ-A 9,823,371 discloses a process for preparing acetic acid by gas-phase oxidation of unsaturated C.sub.4 -hydrocarbons using a coated catalyst comprising a support body and a catalytically active mixed oxide composition. These mixed oxides are selected from the group consisting of titanium dioxide, zirconium dioxide, tin dioxide, aluminum oxide and vanadium pentoxide applied to the outer surface of the support body. After the reaction, the acetic acid formed is separated off by cooling and precipitation or by absorption in a suitable solvent.
DE-A 19,823,052 describes a process for preparing acetic acid by gas-phase oxidation of saturated C.sub.4 -hydrocarbons and their mixtures with unsaturated C.sub.4 -hydrocarbons. This process uses a coated catalyst comprising an inert, nonporous support body and a catalytically active mixed oxide composition comprising titanium dioxide and vanadium pentoxide applied to the outer surface of the support body. In this process, a gas mixture comprising an oxygen-containing gas and C.sub.4 -hydrocarbons together with water vapor is reacted over the coated catalyst at a temperature of from 100.degree. C. to 400.degree. C. and a gauge pressure of from 1.2.times.10.sup.5 to 51.times.10.sup.5 Pa. No method of isolating the acetic acid and further useful materials formed during the gas-phase oxidation is indicated.
DE-A 19,823,088 describes a process for preparing saturated carboxylic acids having from 1 to 4 carbon atoms by gas-phase oxidation in the presence of saturated and/or unsaturated C.sub.4 -hydrocarbons, an oxygen-containing gas and water vapor and in the presence of at least one catalyst. In this process, part of the gas leaving the reactor is recirculated in a reaction gas circuit. This circuit is designed so that part of the organic acids formed in the gas-phase oxidation is removed from the gas leaving the reactor. However, only uneconomical methods such as partial condensation of the gas mixture or rectification processes, if desired with the addition of auxiliaries (e.g. extractive rectification), are indicated for separating off the crude acid.